Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Federal Funds Sold and Securities Borrowed or Purchased under Agreements to Resell [Member] | Assets [Member]
Offsetting Assets [Line Items]
Net amount	$ 427	$ 568
Gross amounts recognized	427	568
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	427	568
Gross amounts not offset in the balance sheet	0	0
Cash Collateral Received/Pledged	0	0
Derivative Financial Instruments, Assets [Member] | Assets [Member]
Offsetting Assets [Line Items]
Net amount	31	47
Gross amounts recognized	46	64
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	46	64
Gross amounts not offset in the balance sheet	(15)	(17)
Cash Collateral Received/Pledged	0	0
Assets, Total [Member] | Assets [Member]
Offsetting Assets [Line Items]
Net amount	458	615
Gross amounts recognized	473	632
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	473	632
Gross amounts not offset in the balance sheet	(15)	(17)
Cash Collateral Received/Pledged	0	0
Federal Funds Purchased and Securities Sold under Agreements to Repurchase [Member] | Other Liabilities [Member]
Offsetting Assets [Line Items]
Net amount	4,342	827
Gross amounts recognized	4,342	827
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	4,342	827
Gross amounts not offset in the balance sheet	0	0
Cash Collateral Received/Pledged	0	0
Derivative Financial Instruments, Liabilities [Member] | Other Liabilities [Member]
Offsetting Assets [Line Items]
Net amount	8	25
Gross amounts recognized	38	59
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	38	59
Gross amounts not offset in the balance sheet	(15)	(17)
Cash Collateral Received/Pledged	(15)	(17)
Liabilities, Total [Member] | Other Liabilities [Member]
Offsetting Assets [Line Items]
Net amount	4,350	852
Gross amounts recognized	4,380	886
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	4,380	886
Gross amounts not offset in the balance sheet	(15)	(17)
Cash Collateral Received/Pledged	$ (15)	$ (17)